Novel package pushes power up and cost down

Vicor has introduced a 1.2kW dc-dc converter with a 61x23mm footprint (x7.3mm), the first product based its novel ‘ChiP’ packaging technology, revealed at APEC 2013.

The package sits right at the top of power density capability, with the firm claiming 183W/cm3 (3kW/in3) and 131W/cm2 (850W/in2) are possible.

Its manufacture is analogous to chip-scale packaging.

Based around a one large double-sided PCB laid out for many converters, it is populated and over-moulded before being sawn into separate converters.

Only after sawing are legs or other terminations soldered to the exposed PCB edges, and protective end caps bonded over the terminations.

Largely the same tooling can be used for any sized package – Vicor is offering six options, from 13×23 to 23x61mm – with only the sawing distance changing.

According to Vicor product line director Gary Hill, ChiP packaging has both technical and financial advantages compared with encapsulation after separating individual converters from the master PCB, as used in the firm’s previous-generation ‘VI Chip’ package.

“What is really important for us is that over-moulding the entire PCB cuts down manufacturing cost,” he said. “VI Chip has great traction in military and supercomputers. Now we are gaining traction in data centres, automotive applications, and performance industrial.”

Automotive application with no airflow and no fan.Cooling liquid flow is through the two holes.

The main technical advantage is that ChiP inherently allows the inductor core to be exposed on both the top and bottom of the package – making double-sided cooling possible. It also allows many component to be positioned on either side of the internal board, which has shortened traces and reduced parasitics, said Hill.

On both sides of the package, to allow heat to be pulled from the semiconductors, the moulding compound is flush with the inductor cooling face (see top image).

“Mosfet heat comes through because the moulded compound is quite thermally conductive. It has a similar thermal conductivity to the inductor,” said Hill. “For customers who fully-embrace double-sided cooling, full power dissipation is available at quite high temperature.”

He added that an application note on double-sided cooling will be released soon.

While fully-regulated ac-dc converters are in the pipeline, the first product using ChiP is a fixed-ratio isolated dc-dc converter implemented with the firm’s ‘sine amplitude converter’ (SAC) topology (see circuit), switching at 1.25MHz and called BCM380y475x1K2A30.

“It is our best example of a resonant switcher. There is not much around in terms of output resistance and it has very very low resistance fets,” said Hill. “Efficiency is 98% at a little over half power. Our previous bus converter had 95-96% peak efficiency.”

The efficiency curve is fairly flat, and typically above 97.5% from 30 to 100% load. Peak possible load is 1.5kW.

Applications are expected in data centres with 380Vdc rail power distribution. “There is an industry body for 380V distribution and an ETSI standard. NTT is pushing it in Japan,” said Hill. “The isolated 380Vdc converter can also be used with non-isolated power factor correction to make an ac-dc converter.”

As it only wastes 2.5%, there “is only around 30W dissipation at full power. It doesn’t require any exotic cooling”, said Hill. “It has a relatively large surface area. At 600W it will cool with just airflow and through its leads.”

Double-sided fan cooling

With double-sided cooling, full-power operation is possible at 100°C, dropping to 70°C with one sided cooling, which can be pushed up to 80°C if heat is pulled into the PCB through the leads.

Even without the app note, some thermal design information is included on the data sheet, said Hill.

By coincidence, Vicor’s fixed ratio SAC topology is bi-directional, a characteristic that has found it additional sales. “It is just as efficient in both directions. We have had a lot of requests. Battery manufacturers can charge a battery, then discharge it with the same converter, recycling the power, not wasting it,” said Hill.

Standard features protect against: under-voltage, over-voltage, over-current, short-circuit and over-temperature. There is also configurable digital telemetry and control.

There is a demo board (BCD380x475x1K2A30), and some on-line tools including an power architecture tool called PowerBench Whiteboard, which is based on actual test data, said Hill.